Generally, the life span of oil, e.g., used in the engine of a vehicle, depends on various conditions, such as the running distance of the vehicle, operating time of the engine, driving environment, and so on.
In most cases, because of physical and chemical degradations, the oil should be replaced regularly. The physical degradation of the oil is usually caused by metallic wear particles and dusts introduced into the oil from outside. These contaminants cause excessive wear on the surfaces of engine components, like cylinders, bearings, cylinder walls, and pistons, thereby deteriorating engine performance considerably. In addition, the chemical degradation is predominantly caused by moisture, degradation products, coolants and foreign oil types flowing into the oil.
There are a variety of methods for determining the condition of oils, but most of these techniques are based on debris monitoring and viscosity. These measurements provide insight into the physical changes that have occurred in the oil (i.e. the presence of metal debris, an increase in viscosity, the presence of water), but provide no direct insight into the chemical changes that have occurred (i.e. the breakdown of an additive package or loss of a specific chemical functionality). In many cases, detecting the chemical degradation is more useful for measuring the oil degradation accurately.
Most engine oils contain various additives to improve the performance of the base oil and slow down the degradation process. Engine oil is not only subject to continually high temperature conditions, but also continuous oxidized when the oil is contacted with water or air. Subsequently, the additives are also deteriorated. Accordingly, various degradation compounds and contaminants are accumulated in the oil. As a result, the viscosity increases, sludge is generated, and oxidized acid compounds easily corrode mechanical components.
Many attempts have been made to measure the quality of oil by various analysis methods. The total acid number measuring method, as described in ASTM (American Standard Test Method) D664, is one method for quantitatively analyzing oil. Other measuring methods also have been developed, such as the total alkali number measuring method (ASTM D4739), viscosity measuring method (ASTM D445/446), gas chromatography of diesel fuel diluents in used diesel engine oil (ASTM D3524-90), flash point measuring method (ASTM D92), oxidation measuring method (ASTM D2272), and so on. However, the above ASTM methods require extracting a sample of the oil from the mechanical device, e.g., the engine, and pre-treatment processes, involving much time and cost. Furthermore, the sample cannot be certain to really represent the status of the whole oil as used in the mechanical device.
Conventional oil analysis methods are usually based on analysis of physical and chemical characteristics of the oil, such as viscosity, dielectric constant, AC conductivity, resistance, impedance, corrosion, pH value, total alkali number, acid material content, spectrum analysis, optical density, and the like.
For example, a method for determining the viscosity of oil by measuring an acoustic transit time change and a phase shift in the oil is disclosed in U.S. Pat. No. 4,721,874 to Emmert. As the oil is oxidized, carboxyl-acid components are generated and the viscosity increases. However, this method has a serious disadvantage because viscosity cannot be measured correctly when the oil is diluted with low viscosity liquids, such as gasoline and water.
U.S. Pat. No. 5,929,754 discloses an engine oil deterioration sensor, which measures the dielectric constant of the oil. The dielectric constant of engine oil is typically between 1.6 and 3.2, depending upon the brand and the period of use. The dielectric constant increases with the period of use. Thus, the dielectric constant of engine oil provides an indication of the oil degradation. However, a mechanical failure, such as a damaged head gasket or a broken piston ring, will change the purity of the oil because contaminants, such as coolant (glycol ethylene), fuel, and water, can easily be introduced into the engine oil. Water and engine coolant have dielectric constants of approximately 87.5 and 37.0, respectively. The introduction of such contaminants into the engine oil significantly affects the measured dielectric constant of the engine oil.
U.S. Pat. No. 5,200,027 discloses a method for measuring the oil condition by using the difference between the conductivity of the oil and that of the contaminants in the oil. However, such a method may only be applied where the primary contaminant is an acid material or water.
U.S. Pat. No. 5,274,335 discloses a device for qualitatively determining the oil condition. The device is composed of two inert metal plated iron electrodes, for example gold plated, and the gap between the two electrodes is filled up with test engine oil. Since the electrical conductivity of engine oil, in general, is extremely low, the two electrodes have to be closely spaced to lower the ohmic resistance. In this device, this distance is set at 0.015 cm. A triangular waveform is applied to the electrodes with, for example, a maximum potential of 5 volts and a minimum potential of −5 volts. The output current increases gradually with the age or degradation of the engine oil. Since an oil's acid number will increase with use, the current increase may be associated to the electrochemical reaction involving the acidic decomposition products of engine oil. Therefore, the device can be used to monitor the oil condition and signal the need for an oil change. However, this method has a serious drawback in that the output current is greatly affected by fuel or coolant contaminants introduced into the oil, as well as the oil's acid number.
Molecular spectroscopy, such as a fourier transform infrared spectroscopy, is also widely used for analyzing the oil conditions. Molecular spectroscopy is based upon the phenomenon that a molecule absorbs the light energy of a specific frequency band, which is called the resonance frequency. By using this phenomenon, the existence of various materials in the oil, such as water, fuel, coolant (glycol ethylene), soot, additives, and so on, can be detected.
Infrared spectroscopy emits infrared light onto the oil sample in a cell and measures the amount of infrared light absorbed by the oil sample. An infrared spectrum is obtained by classifying the power of the infrared light passing through the oil sample by frequencies, thereby indicating which kinds of molecules exist in the oil sample. However, used engine oil commonly contains so many different molecules, additives, decomposition products, metallic wear particles, contaminants and the like, that a very complicated infrared spectrum is obtained. Thus, reliably analyzing the infrared spectrum and determining the kinds of molecules contained in the used oil are difficult.
Fluorescence spectroscopy is also used for measuring the degradation of engine oil. Generally, as the oxidation of the oil progresses, the power of the fluorescence increases.
In the case of car engine oil, aromatic hydrocarbons, polyphenyl hydrocarbons, and carboxyl compounds in the oil are known as organic fluorescent materials. Representative aromatic hydrocarbons include pylene, fluorene, etc.
Generally, mineral oil consists of iso-paraffin, naphthene, aromatic hydrocarbons, and naphtheno-aromatic hydrocarbon compounds. Aromatic hydrocarbons and naphtheno-aromatic hydrocarbon compounds have fluorescent effects due to benzene ring.
A synthetic base oil consists primarily of synthesized hydrocarbons, esters, ethers, halogenated compounds, and silicon polymers, and consists additionally of sodium potassium eutectics or inorganic polymers of boron, phosphorus and nitrogen. These most materials have fluorescent effects.
However, conventional fluorescence spectroscopy requires a considerably narrow oil sample path in order to minimize the light-darkening effect and expensive spectrum analyzing means. Thus, conventional fluorescence spectroscopy is not adapted for use in being mounted to a mechanical device, such as a car engine, and measuring oil degradation in real time. Also, contaminants in the oil tend to adhere to the surface of the oil sample path, thereby deteriorating the operational reliability.
Therefore, the each of the conventional methods used to measure oil oxidation levels have one or more drawbacks.